Electronics such as information processing devices and communication equipment generally include circuit boards inside. A circuit board typically includes an insulating-material substrate and a conductive-material layer on the substrate, and the conductive-material layer includes circuits to form a predetermined circuit pattern. Various electronic components can be mounted on the circuit board by means of processing such as soldering. Recently a multilayer circuit board having a plurality of conductor layers has come to be widely used.
There has been known a conventional circuit board including a polyimide as an insulating material, for example, a circuit board that comprises (i) a wiring substrate comprising a polyimide film and a circuit formed from a conductor layer on the polyimide film and (ii) a coverlay comprising a polyimide film and an adhesive layer to be bonded to the wiring substrate.
However, such a circuit board sometimes has poor heat resistance, especially poor solder heat resistance, due to usage of an adhesive. Further, some circuit boards contain residual solvent originated from the adhesive. The presence of the solvent in the circuit boards may cause defects in the circuit board after multi-layering process, resulting in deterioration of reliability of the circuit boards. Accordingly, a technique for forming a circuit board without using an adhesive has been desired.
In recent years, there have been remarkable developments in the field of information processing, such as personal computers, as well as in the field communication equipment, such as mobile phones. Such electronics and communication equipment have come to be operated at higher frequencies of gigahertz region. In the high frequency band, however, it is known that the electronics and communication equipment generally have increased in transmission loss.
Circuit boards have been conventionally known as one comprising a wiring substrate in which a conductor circuit is formed on a polyimide film, and a coverlay film bonded on the wiring substrate, the coverlay film comprising a polyimide film and an adhesive layer.
However, such a circuit board sometimes has poor heat resistance, especially poor solder heat resistance, due to adhesive usage. Further, some circuit boards contain residual solvent originated from the adhesive. The presence of the solvent in the circuit boards may cause defects in the circuit board after multi-layering process, resulting in deterioration of reliability of the circuit boards. Accordingly, a technique for forming a circuit board without using an adhesive has been desired.
On the other hand, TLCP films have attracted attention as substrate materials for forming circuit boards without using an adhesive. The TLCP film, however, contains rigid skin layers on the surfaces, the skin layers generated during extrusion. Where the TLCP films are heat-bonded with each other, the skin layers sometimes interrupt sufficient interlayer adhesion between the TLCP films.
In order to improve adhesive property, for example, Patent Document 1 (JP Laid-open Patent Publication No. 2010-103269) discloses a method for producing a multilayer circuit board including: extruding a thermoplastic liquid crystal polymer (TLCP) capable of forming an optically anisotropic melt phase to form a TLCP film, softening at least one surface of the TLCP film by physical grinding or UV radiation to render the film surface to have a hardness of 0.01 to 0.1 GPa measured in accordance with the nanoindentation method so as to form an adhesive surface, and counterposing the adhesive surface on a circuit surface of a wiring substrate comprising a conductor circuit on at least one surface of a TLCP film capable of forming an optically anisotropic melt phase and carrying out a thermo-compression bonding of the entire components.
In the meanwhile, where a circuit board is produced without usage of an adhesive by laminating a conductor layer of a metal such as copper to be bonded to a LCP layer, a process for accomplishing an improved peel strength (strength against peeling) has been carried out by forming an uneven surface on the conductor layer to enhance compressive adhesion between the conductor layer and the insulating layer by an anchoring effect of the uneven surface. The optimization of the uneven structure has been studied.
For example, Patent Document 2 (WO 2012/020818) discloses a metal-clad laminate having a metal foil on one or both surfaces of a liquid crystal polymer layer, wherein the metal foil has projections on a surface layer portion on a side to be in contact with the LCP layer, the projection being formed by roughening the metal foil surface; the projections have an aspect ratio (H/L) of a projection height H with respect to a projection bottom width L in a range of 3 to 20; the projection height is in a range of 0.1 to 2 μm; and the LCP layer has a thickness of 10 to 2000 μm and a thickness tolerance of less than 6%.